Specific Salt Effect on the Solubility of fac-Tris(α-amino acidato)cobalt(III)

The salt effect on fac-tris(aniono)cobalt(III) of α-amino acids (fac-[Co(amino acidato)3]), such as D-proline and glycine, was examined by determining its solubility in various aqueous salt solutions. For a comparison, the solubilities of tris(2,4-pentanedionato)cobalt(III) ([Co(acac)3]) and L-phenylalanine (L-phalaH) were also determined. The enthalpy (ΔHs) and entropy of dissolution (ΔSs) were estimated from the temperature dependence of the solubility. While NaCl and Na2SO4 cause a decrease in the solubilities (salting-out effect) of [Co(acac)3] and L-phalaH, these salts result in an increase in the solubility (salting-in effect) of fac-[Co(amino acidato)3]. The salting-out of [Co(acac)3] is caused by an increase in ΔHs, while the salting-out of L-phalaH results from a decrease in TΔSs. On the other hand, the salting-in of fac-[Co(amino acidato)3] is caused by a decrease in ΔHs. These results suggest that the origin of the salt effect on fac-[Co(amino acidato)3] and L-phalaH differs from that of the sa...